The study of the differentiation of human lymphoid cells "in vitro" will be conducted with lymphoblastoid lines. Clones of these lines will be prepared in which the process of differentiation does not happen spontaneously; these clones are appropriate for the study of differentiation-inducing agents (small molecules such as HMBA or DMSO or T-cell factors). The differentiation process will be monitored by detecting immunoglobulin synthesis and secretion. The role of membrane immunoglobulins in the regulation of mouse B immunocytes will be studied "in vitro" with the use of anti-immunoglobulin antibodies (including anti-IgD) and polyclonal B cell activators (such as LPS or Dextran SO4) or specific antigens. Antibodies directed against "individual" specificities of immunoglobulin molecules (idiotypes) will be used to study the role of membrane immunoglobulins in the regulation of specific clones of B immunocytes, and in particular to probe the interactions between T and B lymphocytes at the idiotype level.